Devices are known that wire available to a wire-working machine, e.g. a device that produces wire comb binding elements for binding stacks of sheet-shaped materials.
In such devices that fabricate workpieces or semi-finished products of wire or use welding wire, the wire is drawn from a wire spool. A user of such a device can frequently only recognize the fact that the wire is approaching the end by glancing at the wire spool. In some applications, these spools are either poorly accessible or there are a large number of spools so that it is complicated for the user to manually check the fill level of the wire spool.
Visual inspection does not however, provide the user with accurate information on how many meters of wire still remain on the wire spool and for how many welds, workpieces or semi-finished products the wire will be adequate.
Devices for detecting spooled wire typically use electrically resistive methods in which current is conducted through the wire by means of a contact to determine the length of the wire based on the resistance of the wire that can be measured in this process. These devices are not useful though for wire coated with plastic as is frequently used for wire comb binding elements.
Other devices use optical means in order to determine the remaining quantity of wire. Such methods are typically susceptible to soiling and in addition require to some extent complicated and thus expensive optics.
Serious damage can occur to machinery utilizing the wire if the wire is allowed to run out while the machine is still running. Also, premature changing of the wire spool wastes wire.
Canadian patent CA 1 270 502 discloses a magnet inserted in a spool body which is in active relationship with a magnetic sensor that is mounted outside the spool wall and is opposite the magnet once at each rotation of the spool. The magnetizing of the magnet is such that the magnetic field lines through the cable windings are guided above the magnet, i.e. at a right angle to the winding direction of the cable on the cable drum. The continuous conduction of the magnetic field lines, i.e. of the magnetic flux through the adjacent cable windings, functions especially well if the cable is not coated. Between the magnet and the magnet sensor, flush with the magnet, an intermediate element is embedded in the spool body, this element being made of a material that has permeability that is lower than that of the cable. If no cable is now located above the magnet, the magnetic field lines are conducted through this intermediate element in the area of the magnet sensor so that it generates a signal. This signal correlates with the absence of cable and can, therefore, be evaluated as information that only a specified number of windings remain on the wire spool.
Japanese publication JP 60015360 discloses a device for remaining wire quantity recognition for welding wire, which also uses a magnetic principle. In this process, a magnetic proximity switch and a permanent magnet are mounted on a movable contact arm. The contact arm remains in contact, by way of an insulating element, with the wire; when wire is present a signal is caused in the magnet sensor because of the permanent magnet. As soon as no more wire is present, the magnet sensor switches off so that a signal is generated to indicate that only a little wire is left on the wire spool. Efforts to improve such systems have led to continuing developments.